JPH08127814A - Heat treatment for steel - Google Patents

Abstract

PURPOSE: To perform heat treatment of steel without decarburizing and carburizing by controlling the addition amount of hydrocarbon gas based on the concentration of methane in the exhaust gas exhausted from a heat treatment furnace. CONSTITUTION: A work W of low carbon steel, etc., is held in a heat treatment furnace 1 and nonoxidizing gas is introduced, and heating treatment is done in this atmosphere. At this time, nitrogen gas is supplied via a nitrogen gas supply route 7 while raising the temp. in the heat treatment furnace 1, and the atmosphere in the furnace chamber is replaced with the nitrogen gas. When the temp. in the furnace 1 reaches the prescribed temp., the flow rate of the nitrogen gas to be supplied into the heat treatment furnace 1 is reduced, and hydrocarbon gas, e.g. propylene gas, having 2-4 carbon atoms and double bond is added to the nitrogen gas via a gas supply route 8 and supplied into the heat treatment furnace 1. The additive quantity of this hydrocarbon gas is controlled based on the methane concentration in the exhaust gas exhausted from the heat treatment furnace 1. By this method, the necessity of a gas generator can be obviated, and cost reduction can be attained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a heat treatment method for steel such as carbon steel and carbon alloy steel.

[0002]

2. Description of the Related Art Conventionally, in heat treatment of steel, the temperature of a heat treatment furnace containing a work is increased while purging with nitrogen gas, and when the temperature in the heat treatment furnace reaches a predetermined temperature, supply of nitrogen gas for purging is stopped and The metamorphic furnace gas is supplied into the heat treatment furnace to maintain the metamorphic furnace gas atmosphere in the heat treatment furnace, the work is heated to a predetermined temperature in the metamorphic furnace gas atmosphere, and the work is held at the predetermined temperature for a predetermined time, The heat treatment was performed at the cooling rate.

[0003]

However, in the conventional heat treatment method, the gas for filling the heat treatment furnace is formed by the gas generated by the shift converter furnace, so that the maintenance of the shift converter furnace is troublesome. Also, in the A1 transformation point region such as spheroidizing annealing, there is a risk of explosion if outside air enters the heat treatment furnace from the agitating motor shaft portion and seal portion during processing, and it is troublesome to secure airtightness in the heat treatment furnace. In addition, it was difficult to control the carbon by the oxygen meter and the carbon dioxide meter by the enrichment method. The present invention provides a method for heat treatment based on nitrogen gas, paying attention to such a point.

[0004]

Means for Solving the Problems In order to achieve the above-mentioned object, the present invention increases the temperature in a heat treatment furnace while purging with nitrogen gas, and when the temperature in the heat treatment furnace reaches a predetermined temperature, it is supplied into the heat treatment furnace. The flow rate of the nitrogen gas to be reduced is reduced, and a hydrocarbon-based gas having 2 to 4 carbon atoms and having a double bond is added to the nitrogen gas and supplied into the heat treatment furnace, and the addition amount of this hydrocarbon gas is changed from the heat treatment furnace. It is characterized in that the atmosphere in the furnace is controlled based on the methane concentration in the exhaust gas discharged.

[0005]

In the present invention, the temperature inside the heat treatment furnace is increased while purging with nitrogen gas, and when the temperature inside the furnace reaches a predetermined temperature, the flow rate of nitrogen gas supplied into the heat treatment furnace is reduced and
A hydrocarbon-based gas having 2 to 4 carbon atoms and having a double bond is added to nitrogen gas and supplied into the heat treatment furnace, and the addition amount of this hydrocarbon-based gas is adjusted to the methane concentration in the exhaust gas discharged from the heat treatment furnace. Since it is configured to control the atmosphere in the furnace based on this, the control of the carbon content in the work uses a hydrocarbon-based gas having a double bond with a carbon number of 2 to 4 that decomposes at a low temperature, and the furnace wall The reaction is performed by reacting oxygen or water that has entered through the gap between the furnace wall and the furnace gas with the hydrocarbon gas to generate carbon monoxide.

[0006]

EXAMPLE FIG. 1 is a schematic view of an apparatus for annealing using a bell-type furnace. Reference numeral (1) is a heat treatment furnace formed in the bell-type furnace, and a stirring fan is provided at the bottom of the heat treatment furnace (1). (2) is arranged, and the fan shaft (3) of the stirring fan (2) penetrates the bottom wall (4) of the heat treatment furnace (1) and projects outside the furnace.

At the bottom wall (4) of the heat treatment furnace (1), an atmosphere gas inlet (5) and an atmosphere gas outlet (6) are formed, and the atmosphere gas inlet (5) is The nitrogen gas supply passage (7) and the addition hydrocarbon gas supply passage (8) are joined together and connected. The nitrogen gas supply channel (7) has an on-off valve (9) with adjustable flow rate.
And a nitrogen gas source (1
1), and the addition hydrocarbon-based gas supply passage (8) is connected to the addition hydrocarbon-based gas container (14) via the flow rate control valve (12) and the flow meter (13). . In addition, propylene is used as the carbon-hydrogen-based gas for addition.

On the other hand, an orifice (16) is arranged in the exhaust gas passage (15) from the atmosphere gas outlet (6), and the orifice is branched from the upstream side of the orifice (16) for analysis. A gas outlet pipe (17) is connected to the analyzer (18). Then, based on the methane gas concentration analyzed and detected by the analyzer (18), the opening / closing degree of the flow rate control valve (12) arranged in the addition hydrocarbon-based gas supply passage (8) is adjusted.

The procedure of the annealing operation in the heat treatment furnace having such a structure will be described below. The work (W) to be treated is set in the heat treatment furnace (1). At the time of setting this work, the inside of the furnace is in an atmospheric atmosphere, and the atmosphere and water vapor have entered the inside of the furnace wall. And if the temperature is raised as it is, the surface of the work will be oxidized by the influence of oxygen in the atmosphere,
While heating the inside of the heat treatment furnace (1), nitrogen gas is supplied from the nitrogen gas supply passage (7) to replace the atmosphere in the furnace chamber with nitrogen gas.

When the inside of the furnace becomes a nitrogen atmosphere having a residual oxygen concentration of about 100 ppm and the temperature inside the furnace reaches about 550 ° C., which is the decomposition temperature of propylene, the on-off valve (9) of the nitrogen gas supply passage (7) is throttled. In the heat treatment furnace (1), reduce the flow rate by half and open the flow rate control valve (12) of the hydrocarbon gas supply passage (8) for addition to adjust the propylene gas to 1% nitrogen gas. Supply to.

Based on the heat treatment pattern shown in FIG. 2, the work was kept at a temperature about 20 ° C. higher than the A1 transformation temperature for about 10 hours while continuously supplying the nitrogen gas added with 1% of propylene gas, and then slowly. And cool
Anneal the work.

[0012] During this high temperature maintaining period, a small amount of outside air enters from the portion where the stirring fan shaft (3) is inserted, but at this time,
When an oxidizing gas such as oxygen or water exists in the furnace, propylene reacts as follows to generate carbon monoxide gas and hydrogen gas, and prevents decarburization of the work.

Embedded image

On the other hand, when the amount of oxidizing gas in the furnace is exhausted during the high temperature maintaining period, propylene decomposes as follows to produce methane gas.

Embedded image When the amount of carbon increases, the work surface is carburized.

Therefore, the amount of methane gas in the exhaust gas is analyzed and detected, and the opening amount of the flow rate adjusting valve (12) in the addition hydrocarbon gas supply passage (8) is adjusted based on the amount of methane gas,
Adjust the amount of propylene gas added to nitrogen gas. And, depending on the airtightness of the furnace, when the amount of propylene gas added exceeds 2%, the above formula is decomposed, carburization is observed on the surface of the work, and free carbon is observed to adhere to the furnace wall. It was Further, when the amount of propylene added fell below 1%, decarburization was observed on the surface of the work. As a result, it is desirable to control the addition amount of propylene to 1 to 2% in the annealing work. In addition, carburizing treatment or decarburizing treatment can be performed by adjusting the addition amount of propylene.

Although propylene gas is used as the hydrocarbon to be added in the above embodiment, ethylene gas may be used. In this case, a hydrocarbon that decomposes at a temperature lower than the heating temperature of the work is required. Therefore, a hydrocarbon having a double bond that easily decomposes with 2 to 4 carbon atoms is optimal, but propane, ethane, or the like. It may be a hydrocarbon having no double bond.

[0016]

According to the present invention, the temperature in the heat treatment furnace is raised while purging with nitrogen gas, and when the temperature in the furnace reaches a predetermined temperature,
While reducing the flow rate of nitrogen gas supplied into the heat treatment furnace, a hydrocarbon-based gas having a carbon number of 2 to 4 and having a double bond is added to the nitrogen gas and supplied into the heat treatment furnace. Since the addition amount is controlled based on the methane concentration in the exhaust gas discharged from the heat treatment furnace,
The amount of carbon in the work can be controlled by reacting oxygen and moisture remaining in the furnace with hydrocarbon-based gas to control the amount of carbon in the work. It can be heat treated without carburizing or decarburizing or carburizing. Moreover, in that case, since only a small amount of hydrocarbon gas is added to nitrogen gas, it is superior in safety compared to the one using conventional metamorphic furnace gas, and only the part that does not require a metamorphic furnace is used. The structure is simplified,
The maintenance cost of the converter is not required, so the running cost can be reduced.

[Brief description of drawings]

FIG. 1 is a schematic view showing an example of an annealing device.

FIG. 2 is a time-temperature relationship graph showing a heat treatment pattern.

[Explanation of symbols]

 W ... Work, 1 ... Heat treatment furnace.

Claims (4)

[Claims] 1. A work made of low carbon steel or low carbon alloy steel.
In a heat treatment method for steel in which a non-oxidizing gas is injected into the heat treating furnace (1) containing (W) and heat treatment is performed in a non-oxidizing gas atmosphere, the heat treating furnace (1) is purged with nitrogen gas. When the temperature rises and the temperature inside the furnace reaches the specified temperature, the heat treatment furnace
While reducing the flow rate of nitrogen gas supplied to (1),
Exhaust gas discharged from the heat treatment furnace (1) by adding a hydrocarbon gas having 2 to 4 carbon atoms and having a double bond to nitrogen gas and supplying the gas into the heat treatment furnace (1) A method for heat treatment of steel configured to be controlled based on the concentration of methane in the steel. 2. The steel according to claim 1, wherein the work (W) is heat-treated without decarburization or carburization by controlling the addition amount of the hydrocarbon gas to the nitrogen gas to be 1 to 2%. Heat treatment method. 3. The heat treatment method for steel according to claim 1, wherein the surface of the work (W) is decarburized by controlling the amount of the hydrocarbon gas added to the nitrogen gas to be less than 1%. 4. The heat treatment method for steel according to claim 1, wherein the surface of the work (W) is carburized by controlling the amount of the hydrocarbon gas system added to the nitrogen gas to 2% or more.